I was engaged in research on optical fiber sensors at graduate school. An optical sensor detects changes in reflection components of light propagating in an optical fiber to measure continuous distribution of data along the fiber, such as temperature and tensile strain, accurately in a real-time basis. I studied the subject because I found the properties of the lightwave scientifically interesting. Having completed the Master's program, I chose a path to become a researcher in the private sector, like most Japanese researchers and engineers do so, with an idea that research activities in a field outside of academia would foster my growth.

While I was engaged in the research and development of optical fiber sensors after joining Sumitomo Electric, which was an extension of my research at graduate school, I sometimes felt unsatisfied with the development activities with predictable results. I had a desire for something unknown and new to me. Just then, I was assigned to research optical fibers themselves. I worked in research and development for the improvement of the performance of existing optical fibers for communication, which led to research on a multi-core optical fiber (MCF), the subject to which I am still committed.

An optical fiber is a very thin line to transmit optical signals. The optical fiber dramatically improved the speed and capacity of data transmission, underpinning the sophisticated information society in the modern world as represented by the Internet. Nevertheless, with the introduction of various network services such as smartphones and video distribution services, the volume of data flowing through the network has been growing each year. While continuous improvements have been made to expand the transmission capacity based on wavelength division multiplexing* and other innovative technologies, the transmission capacity of the conventional single mode optical fiber (SMF) is getting close to the limit.

In the SMF, the central part called the "core" is covered with a layer called "cladding" concentrically and light is confined in and transmitted through the only one mode (path for light) that exists in the core. To overcome the limits of the SMF, research on space division multiplexing was started to add more paths for light in an optical fiber. In this research trend, I focused on the MCF in my research and development activities. While a conventional optical fiber has a core in the cladding, the MCF has multiple cores in a cladding and is expected to dramatically increase the transmission capacity. On the other hand, there is a concern that the existence of multiple cores causes characteristic degradation that does not occur in the SMF, and the suppression of the degradation was a major challenge for practical use.

* Wavelength division multiplexing: technology to transmit multiple signals with light of respectively different wavelengths in order to increase the transmission capacity of an optical fiber

One of the problems was the core-to-core crosstalk (XT), which is mutual interference of signals between cores that degrades the quality of communication. Through experiments, I found that much larger XT than predicted based on the conventional theory could be generated, and discovered and demonstrated that it was attributed to a phase matching in the light between cores caused by bending the fiber. I then developed a method to suppress XT by conversely using the fiber bends to induce the phase mismatch in light, and we realized an ultra-low-XT MCF which still has the lowest XT record as MCFs applicable to long-haul transmission, to the best of our knowledge. Using the ultra-low-XT MCF, the first transmission experiment achieving more than 100 Tb/s/fiber—regarded as the SMF capacity limit— was demonstrated in collaboration with the National Institute of Information and Communications Technology and others in 2011. Recently, in collaboration with KDDI Research, Inc., the transmission experiment with an ultra-high capacity of more than 10 Pb/s—100 times larger than the SMF capacity limit (100 Tb/s)— over a few-mode MCF was successfully demonstrated, by increasing the number of modes in each core to realize mode division multiplexing in addition to suppressing the core-to-core XT.

We have also collaborated with Bell Laboratories (now Nokia Bell Labs) to establish an approach that allows XT and recovers the data at the receiver. The allowance of XT enables cores to be located closer to each other, which has an advantage of transmitting a larger volume through a fiber of the same diameter with that of a conventional standard optical fiber. On the other hand, arrival time differences between the signals (modal dispersion) were a major challenge because they have a large impact on the calculation complexity for recovering the information. Then, we fabricated a coupled MCF in a new original design and significantly suppressed the modal dispersion, which is low enough for practical ultra-long-haul transmissions. As the coupled MCF design was compatible with the manufacturing process of the ultra-low-loss optical fiber, where Sumitomo Electric excels, the transmission loss has also been reduced to the level equivalent to that in a high-quality ultra-low-loss SMF thanks to the efforts of our manufacturing engineer.

Thus, a coupled multi-core optical fiber suitable for ultra-long-haul transmission with excellent optical properties was realized with a standard cladding diameter of 125 μm. I believe that I won the prize from the Optical Society because our development demonstrated a practical and realistic solution to support future growth of communication traffic. This was a milestone for me as a researcher and I am now working to realize the practical use and mass production of those research outcomes.

I always keep it in mind as a researcher that when I encounter a new phenomenon or challenge, I will review the relevant accepted theories, papers, and books to find any oversight or omission in them and think it out until I can understand it and become convinced.

Research involves constant encounters with something unknown or new and clarification of it. You may face a huge barrier. To overcome it, you have to think thoroughly and care about things in a persistent manner. This is my style as a researcher. Sticking to the professional style, I hope to grow to be a researcher who can promote technological progress, and create technologies and products that widely support society.

Tetsuya Hayashi

2006
Entered Sumitomo Electric Industries, Ltd. Assigned to Optical Communications Laboratory and has been consistently engaged in the research and development of optical fibers up to the present.

2009
Started research on multi-core optical fibers.

2013
Received a Ph.D. for research on multi-core optical fibers.

2016
Became the third winner of the Tingye Li Innovation Prize* in OFC from the Optical Society (OSA) for his work on a coupled multi-core optical fiber suitable for ultra-long-haul transmission.

* The Tingye Li Innovation Prize is awarded each year to a young professional at the age of 39 or younger who has demonstrated the most innovative ideas during the Optical Fiber Communication Conference (OFC), the world's largest international conference on optical communication, and at the Conference on Lasers and Electro-Optics (CLEO), one of the largest international conferences on lasers and optical electronics in the world.